The present invention relates to a pump-injector unitary assembly to achieve both fuel metering and fuel injection in an internal combustion engine.
Fuel metering pumps are already known in the art and for example described in Britist Pat. No. 139,742.
From U.S. Pat. No. 3,131,866 and the paper "simulation of the Cummins Diesel Injection System" of Andrew Rosselli and Pat Badgley presented to the Society of Automotive Engineers No. 710,570, there is already known a fuel injector comprising an injector body provided with an axial bore whose bottom is traversed by at least one fuel spray nozzle.
An inlet for pressurized fuel opens in this bore near the bottom thereof, this duct being connected with a fuel inlet circuit. A plunger or needle is slidably mounted in this bore between a first or upper position, where this plunger is spaced from the bore bottom, and a second or lower position at the end of the injection, where the tip or tapered end of this plunger obturates the fuel spray nozzles by contact with the bottom of the bore.
The displacements of the plunger are controlled by the assembly of a cam a push rod and a rocker-arm, against the action of a return spring.
When using such an injector, the injected fuel charge is adjusted by metering the amount of fuel admitted into the bore through the admission duct.
The bore is filled with a greater or lesser fuel quantity depending on the fuel charge to be injected, at the moment when the plunger begins its downward stroke for discharging fuel.
To this end fuel is supplied to the inlet port of the pump-injector assembly under a pressure varying in relation with the position of the gas pedal and the engine running speed. Thus, the fuel quantity admitted into the bore varies according to the inlet pressure and the duration of the fuel metering period (this period being inversely proportional to the engine running speed), hence the designation of P-T (i.e. Pressure-Time) system.
The drawbacks of such systems are to be found on the one hand in the difficulty of balancing the fuel flow rates delivered by the different injectors in a multi-cylinder engin, due to the importance of an accurate calibration of the fuel inlet port in each injector, and, on the other hand in the methd itself of automatically controlling the injection, through the fuel supply pressure.
Other injection systems are described in the German Patent Application No. 2 719 228 and the French Pat. No. 1 108 081, these systems comprising a pump-type device for transferring the metered fuel quantity into that portion of the injector where the injection nozzles open. In such prior devices gases ar admitted into the injection system and not only the beginning of the injection varies with the fuel charge due to the high compressibility of these gases, but furthermore, the beginnng of the fuel injection cannot be known with accuracy.